Runner blade construction consists of either a solid casting or a fabrication. Most Francis runner blades built in the 70's and 80's were solid cast with extra material added in critical areas for final finishing. These castings were, and often still are, hand ground to templates and fixtures to meet the desired shapes. As machine tools have improved and the runner blade designs have become more computerized, critical areas and features of more complex geometries, such as integral passages, can nowadays be machined into the cast blades. However, this more modern machining approach has done little to improve the manufacturing cycle time, due to the long lead time associated with cast components, nor has it materially reduced manufacturing costs. Although fabricated blades offer improved cycle time and cost advantages, it has proven difficult in practice to fabricate blades having complex geometric features such as integral gas channels, particularly in the thinner portions of the blade.
The ability to construct runner blades with integral passages provides an opportunity to address environmental concerns over the water quality releases from hydroelectric facilities. Hydraulic turbines draw their discharges from the lower level of the upstream reservoir. During the summer months, thermal stratification of the reservoir produces a surface layer of less dense, warm water with relatively high dissolved oxygen and a lower, relatively cold oxygen-depleted layer resulting from the decay of organic material on the reservoir bottom. Thus, turbine discharges during the summer months are often low in dissolved oxygen (DO), which can contribute to water quality problems downstream of the facility.
Relicensing and rehabilitation of an existing hydroelectric facility offers an opportunity to address these concerns over dissolved oxygen levels and other water quality regulations which affect hydropower releases. Rehabilitating an existing hydroelectric facility may include replacement of the runner. Replacing an existing runner with a new runner having integral passages, and providing air through existing structures or new structures of the installation to the integral passages, enhances dissolved oxygen levels in the discharges without material losses in efficiency or substantial increases in cost of rehabilitation.
One effort to construct turbine runners having fabricated blades with integral passages for increasing the level of dissolved oxygen is disclosed in co-pending U.S. patent application Ser. No. 08/733,366, filed Oct. 17, 1996, to the same assignee. In that application, the gas passage is integrally formed in the blade between a leading blade portion and a trailing blade portion. More precisely, a rearwardly facing slot in the leading blade portion cooperates with a forwardly facing slot in the trailing blade portion to form the integral gas passage when the portions are joined. The trailing blade portion also includes a plurality of gas discharge passages extending from the integral gas passage to the trailing edge of the blade.
The ability to fabricate runner blades with integral passages or cavities offers other advantages besides the opportunity to address the water quality of hydroturbine releases. Most importantly, a hollow blade is lighter and saves material costs. The lighter weight of the blades can also improve turbine efficiency. In addition, the hollow blade can be manufactured thicker, which in some cases can also enhance turbine performance and may be better for fish.
The foregoing therefore indicates there is a need for a method of forming a runner blade having gas channels or cavities of relatively complex geometry which offers improved cycle time and is less expensive than the prior art methods.